1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States
2
Now
at Department of Earth Science, University of California Santa Barbara
3
School
of Geography, Earth and Environmental Sciences, Plymouth University,
Plymouth, PL4 8AA, UK
4 Department
of Earth & Environmental Sciences, Columbia University,
Lamont–Doherty Earth Observatory, Palisades, NY 10964, USA
5 School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, European Way, Southampton, SO14 3ZH, UK
6 See
Apendix A
ABSTRACT
Although
ocean crust covers over
60% of Earth’s surface, the processes that form, cool, and alter the
ocean
crust are not completely understood. We utilize shortwave infrared
micro-imaging
spectroscopy of ~1.2km of rock cored by the International Continental
Scientific Drilling Program’s Oman Drilling Project to quantify
hydration of
basalts/gabbros from the Samail ophiolite as a function of depth,
mineralogy,
and deformation. We develop a regression (R2=0.66)
between area of
the ~1350-1650nm OH/H2O absorption and
measurements of loss on ignition
of samples and apply this relationship to generate quantitative ~250
µm/pixel hydration
maps for all cores. The lowest mean hydration is observed in the most
pervasively altered dike-gabbro boundary (GT3A, H2Omean=2.1
wt%), consistent with the fact that a dominant alteration mineral,
amphibole,
has low H2O. The highest H2O
occurs in deeper foliated
and layered gabbros (GT2A, H2Omean=3.2
wt%) and layered
gabbros (GT1A, H2Omean=2.8
wt%). The greater prevalence
with depth of zeolite alteration phases as opposed to lower wt% H2O
amphibole at shallow stratigraphic depths, as well as the occurrence of
zones
of intensive hydration associated with fault zones (H2Omean=5.7
wt%) lead to greater hydration of the lower ocean crust. While
unlikely, some hydration
may be related to obduction. This new approach provides an objective
quantification of hydration in these cores, enabling an improved
understanding
of quantities and characteristics of ocean crust hydration. It
highlights the
importance of specific phases and faulting in controlling hydration,
which has
implications for ocean crust cooling, rheological properties, and the
role of
alteration in global biogeochemical cycling.